Biogas Recovery

Efficiency is all about getting the most output from the least input. One of the best ways to do this is to look at waste and find cost-effective ways of putting it to use. Aside from sending organic material straight to the landfill, the status quo has been to throw it into a compost bin and let the waste decompose into fertilizer. This involves both aerobic (with oxygen) and anaerobic (without oxygen) digestion, in which microorganisms break down biodegradable material. Anaerobic digestion (Figure 1) produces biogas, which is primarily methane and carbon dioxide along with other trace gases. Biogas can be used for heating in boilers and furnaces; it can also generate electricity through a process known as cogeneration (also called combined heat and power). The gas can also be sold, allowing for additional revenue streams, and may qualify for carbon credits. Many industries produce organic material such as plant matter, manure, yeast, and food wastes as part of their waste streams. These are potentially valuable resources that can be recovered and returned to on-site processes for substantial efficiency gains and energy savings.

Figure 1: Anaerobic digestion process

Organic waste is collected, pretreated, and then fed into the anaerobic digester. Biogas is extracted as a product of the anaerobic reaction. The biogas may be used to generate heat or electricity or to produce useful byproducts such as fertilizer, compost, and animal bedding.

What are the options

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A wide range of technologies are available for anaerobic digestion. Digester types are subcategorized based on the type of waste stream being processed: manure, municipal and industrial wastewater treatment, and organic solid waste.

Manure

Anaerobic digestion systems for manure are designed to produce biogas and reduce methane emissions, odors, pathogens, and unwanted seeds. Manure digestion systems fall into four categories:

Covered anaerobic lagoon digester. A sealed lagoon with a flexible cover and piping to transport the recovered methane to be combusted.

Plug flow digester. A long, narrow tank with a rigid or flexible cover, typically built below ground for insulation (thus requiring less supplemental heat); ideal for facilities like dairy farms that can scrape up their manure and place it directly into the tanks.

Complete mix digester. Enclosed, heated tank with a mechanical, hydraulic, or gas mixing system; works best when manure is slightly diluted with wastewater.

Dry digester. Vertical, silo-style tank made of concrete and steel, with rigid covers; has the advantage of operating at a much higher solids content (20 to 42 percent) and requires less liquid for dilution or cosubstrates.

Stored manure can be odorous and may become a breeding ground for pathogens. Biogas is also flammable and must be managed properly. In 2009, the US Department of Agriculture Natural Resources Conservation Service published Anaerobic Digester technical guidelines (PDF) for covered lagoon, plug flow, and complete mix digesters in order to address these issues.

Municipal and industrial wastewater

Anaerobic digesters are used at municipal wastewater treatment plants to break down sewage sludge and eliminate pathogens before effluent is returned to the environment. For industrial facilities, both food and beverage manufacturing facilities typically generate waste streams with high energy potential. They’re known to have high chemical oxygen demand and solids loading. Wastewater treatment digesters fall into three main subcategories:

Mesophilic. The most common type, mesophilic bacteria digesters operate between 70° and 100° Fahrenheit (F), the temperature range in which mesophilic bacteria thrive. Research has shown that mesophilic systems are more stable due to a wider variety of bacteria that grow at mesophilic temperatures, and they’re more adaptable to varying environmental conditions.

Thermophilic. These digesters are run at 122° to 140°F. Thermophilic digestion offers advantages of faster reaction rates and a faster overall process, and it’s also more effective at killing pathogens. This is notably less important if the waste stream goes through pasteurization prior to digestion. Drawbacks include higher expenses because additional heat is needed to maintain the reaction, as well as greater environmental sensitivity and a need for careful temperature control.

Temperature-phased. These digesters offer a combined approach by applying thermophilic digestion for a short period, followed by a mesophilic period. The short thermophilic period helps to kill off pathogens, but these systems do not require as much oversight as a pure thermophilic system.

Organic solid waste

When a municipality is looking at ways to deal with its own waste streams, the solid organic portion (for example, food scraps, yard waste, or paper) becomes a good candidate for anaerobic digestion. This process is a more highly controlled way of capturing methane when compared with landfill gas capture. The organic solid waste stream can be presorted into separate sources to produce a more energy-dense feedstock. System types ideal for handling municipal solid waste include:

Single-stage wet digesters. Simpler and less expensive, these systems are limited by the capability of the organisms to handle the sudden change in pH that occurs during the reaction.

Dry fermenters. A subtype of single-stage digesters, dry fermenters use feedstocks in a solid state, can be handled via front-end loaders without needing additional water, and operate in two modes. In batch mode, all the materials are processed at once; in continuous mode, fresh waste is constantly added and the digested gas is constantly removed.

Two-stage digesters. More complex and expensive, two-stage digestion separates the acid-producing part of the digestion process to allow for more loading of gas from waste that contains lots of nitrogen. Feedstock is normally diluted with water to achieve the right amount of solids content.

Refining biogas

It’s worth noting that biogas is very similar to natural gas and can replace it in most applications. Depending on the type of feedstock used, biogas may need to be refined to remove moisture or hydrogen sulfide, which can corrode combustion equipment. According to Biogas Flares—State of the Art and Market Review (PDF) from the International Energy Agency, combustion temperatures should be kept to 850° to 1,200°F to avoid environmental hazards such as carbon monoxide, dioxins and furans, partially oxidized hydrocarbons, and nitrous oxide. Biogas can be further refined to biomethane by removing carbon dioxide, nitrogen, and oxygen in order to get it to pipeline-injection grade and sold as a pure fuel. The American Biogas Council hosts an interactive biogas processing chart that details the biogas refinement process. Biomethane is also more valuable than both natural gas or biogas due to having a higher direct heating value.

How to make the best choice

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Anaerobic digesters can be installed for many reasons and at many different facility types. First and foremost, it’s important to conduct a technical feasibility assessment to determine if anaerobic digestion is a good choice for your facility.

Determine feedstock type. Even if a feedstock is known to be difficult to digest, it’s possible to add other feedstock types for codigestion, helping to stimulate the reaction. Additional feedstock types can be collected from nearby sources, such as food waste from restaurants or cafeterias, fats and oils from restaurant grease traps, and crop residues from farms. Fats and oils are known to be some of the best feedstocks for yielding the most biogas per volume of feedstock. Many dairy farms will add local food scraps to their manure to increase the amount of biogas produced (Figure 2). AgSTAR, a national partner organization of the US Environmental Protection Agency (EPA), provides assistance and resources for implementing biogas recovery across the US. Its informational sheet on Increasing Anaerobic Digestion Performance with Codigestion describes how to increase performance with codigestion.

Figure 2: Benefits of codigestion

Manure supplemented with feedstocks suitable for codigestion can produce much more methane than manure alone.

Determine size. Sizing requirements vary by application and digester type and depend on feedstock volume and frequency. Sending feedstock to a centralized anaerobic digestion facility can be more economical if close enough to feedstock production.

Ensure feedstock quality. Make sure your organic waste is separated from undesirables such as rocks, straw, and debris. Biogas is a good choice for farms that regularly collect manure. Whether it’s manure or other organic material, waste needs to be fresh because the older it is, the less energy it will contain upon digestion. Energy content varies widely across different kinds of organic waste feedstocks (Table 1).

Table 1: Feedstock energy content

A major benefit of anaerobic digestion is the wide range of feedstocks a digester can receive and process. Fats in particular are known to provide the most energy per feedstock ton.

Assess temperature requirements. In cold climates, facilities may need to provide supplemental heat to drive the reaction. In warm climates, having digester covers exposed to sunlight can be a great option, although keep in mind specific temperature ranges should be maintained for the various digestion types.

Choose how to use the gas

Heating. Biogas can be used in boilers to produce hot water or steam, burned directly for process heat, and used in furnaces for space heating. Boilers are generally able to burn gas without cleaning the gas beforehand.

Electricity via cogeneration. Biogas recovery systems that generate electricity can provide an additional revenue stream for facility owners. In 2016, the EPA published Interconnection Guidelines for connecting biogas generation systems to the grid for the sale of electricity. The heat provided via cogeneration can also be fed back into the digesters to maintain process temperatures. Figure 3 depicts the cogeneration process, where both heat and electricity are recovered from the biogas. See also the Distributed Generation Technologies page, Microturbines, Fuel Cells, and Stirling Engines.

Figure 3: Cogeneration process

Cogeneration involves adding water to a hot gas turbine that is generating electricity. The water heats up from interfacing with the engine and then can be delivered to the facility as water or steam for heating or cooling. The electricity can be used on-site or sold to the local utility.

Sell refined or unrefined. When biogas is conditioned or upgraded into biomethane, it can potentially sell for a higher price than natural gas. This is because biomethane has a higher direct heating value.

Conduct a financial assessment

There are many ways to recover biogas and just as many financial incentives to help encourage adoption. The AgSTAR National Mapping Tool displays updated information on existing digestion projects and state policies and incentives across the US. One of the biggest hurdles for implementing anaerobic digestion systems is the up-front capital cost, but it’s important to take into account all outputs produced by the digestion process. In addition to electricity, separated solids can be sold as animal bedding and compost. In the EPA report, Funding On-Farm Anaerobic Digestion (PDF), Tollenaar Holsteins Dairy notes that these sales generated more revenue than electricity. The same report mentions that, although pricier, codigestion can double electricity sales, with one facility (Butler Farms) estimating $130,600 per year in revenue. For comprehensive planning, AgSTAR hosts a resource hub on Financing Anaerobic Digestion Projects.

Legal requirements and permits

Anaerobic digesters are subject to local, state, and federal permits, which vary considerably from place to place and change frequently. The first step is to contact your local government to determine rules concerning construction, zoning, and storm water management. Then contact your state environmental agency to assess current regulations before starting a digester project. The AgSTAR Guidelines and Permitting for Livestock Anaerobic Digesters offers comprehensive resources on anaerobic digestion, codigestion, and permitting guidelines.

State environmental permits for digesters may be required for air, solid waste, and water. An air permit may be required if the combustion engines are emitting more than federal standards allow. Solid waste processing facilities must meet the federal Resource Conservation and Recovery Act Subtitle D (nonhazardous solid wastes) and 40 CFR Part 258 (landfills) requirements, which are administered through state agencies. Additional solid waste permits may be required if the facility is planning to use codigestion or receive off-site waste. If the anaerobic digesters only process manure, often these are permit-exempt. Finally, a National Pollutant Discharge Elimination System permit (administered through state agencies) is required if the anaerobic digester directly discharges into US waters.

Due to the various types of digesters and feedstocks, it’s imperative to discuss these options with system designers who will know what best suits your facility’s needs. Digester system designers typically take their design cues according to the findings from laboratories that conduct ongoing research for various feedstock types and digester systems.

What’s on the horizon

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According to the American Biogas Council, more than 2,100 sites produce biogas in the US, with the majority being wastewater treatment plants and the minority being farms. American Biogas estimates that more than 11,000 sites are ready for development, most of which are farms. If implemented, these biogas systems could produce enough energy to power 3.5 million homes.

As the industry continues to grow, advances in digester technology and reduction in costs will continue. Smaller digesters are also showing lots of potential to enable cooking directly with harvested biogas, which is promising for residential and restaurant applications.

An emerging alternative to traditional anaerobic digestion is bioelectric wastewater treatment. This method uses electrogenic bacteria to extract clean water and energy from wastewater, showing particular promise for the alcoholic beverage industry. This new biogas technology is especially promising because it can also provide clean water.

Who are the manufacturers?

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AgSTAR stands as the go-to information hub for biogas in the US. Its vendor directory contains lists of designers, project developers, energy service providers, and manufacturers. Major manufacturers include:

Neither this list nor any mention of a specific vendor or product constitutes an endorsement or recommendation by E Source, nor does any content the Business Energy Advisor constitute an endorsement or recommendation, explicit or otherwise, of your service provider’s various technology-related programs.

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